Walls and Gates / Max Area of Island

LeetCode 286 (Walls and Gates) and LC 695 (Max Area of Island). Multi-source BFS for the distance-fill variant; matrix DFS for the area variant. Both surface as standard grid-traversal warm-ups at Meta. Requirements Walls and Gates (LC 286) Grid of INF (empty), -1 (wall), 0 (gate). Fill every empty cell with the distance to the nearest gate. Multi-source BFS from all gates simultaneously is O(mn); single-source BFS per gate is O(mn × #gates) and rejected. Max Area of Island (LC 695) Grid of 0/1; return the area of the largest 4-connected island of 1s. DFS / BFS per unvisited 1, tally cells, take the max. Examples LC 286: a 4×4 grid with two gates fills empty cells with their Manhattan-distance-to-nearest-gate. LC 695: [[0,1,1],[1,1,0],[1,0,1]] → 5. Notes The multi-source BFS pattern (start all sources in the initial queue) is the Meta-signature variant — drill it. Common bug in DFS: not marking visited until after the recursive call returns, causing redundant work. Walls and Gates extends naturally to LC 542 (01-matrix) and LC 994 (rotting oranges). Preparation Write multi-source BFS from memory in under 10 min. Drill LC 286 + 542 + 994 as a triplet (all multi-source BFS). For Max Area, practice both DFS and BFS implementations.